1. Field of the Invention
The present invention relates to a thin film formation method, and more particularly, to a method for forming a metal film, a metal oxide film or a metal nitride film that can be used as an insulating layer, a dielectric layer, or an interconnect layer in a semiconductor or flat-panel display substrate.
2. Description of the Related Art
In the prior art, a metal film, a metal oxide film, or a metal nitride film have been formed by means of a physical vapor deposition (PVD) method such as sputtering. However, PVD is known to have poor step coverage and produce a non-even surface due to an overhang phenomenon where the opening of a hole or a trench is closed before the hole or trench are fully filled. For these reasons, chemical vapor deposition (CVD) methods that form a layer having a relatively uniform thickness with good step coverage has been widely used in recent years.
However, for the conventional CVD technique, the source materials required to form a layer are supplied at the same time, and therefore it is difficult to form a target layer having a desired composition and property. In addition, reaction of a variety of materials used to form a layer occurs in the gas phase, thereby causing the generation of contamination particles. When a multicomponent metal oxide such as barium titanate (BaTiO3) or barium strontium titanate ((Ba,Sr)TiO3) is deposited by CVD, the metals react with each other. As a result, it becomes difficult to form a layer having a uniform composition over a large area on the surface of a 300-mm wafer. In case of a CVD process where an organometallic compound is used as a source material, a large number of carbonic impurities are generated in the resulting film by raising the temperature of the substrate in order to increase the rate of film formation.
As a method of forming a thin film using a chemical compound material, an atomic layer deposition (ALD) method in which reactant gases containing the respective atoms of the compound are sequentially supplied is disclosed in U.S. Pat. Nos. 4,058,430 and 4,389,973 issued to Suntola et al. in 1977. According to the ALD method, the thickness of a layer formed on a substrate by the absorption of reactant gases is almost constant regardless of the duration of reactant gas supply. Thus, the thickness and composition of a target layer can be accurately adjusted by varying the number of pulsing cycles of the source gases. However, ALD methods require separate pulses of reactant gases containing respective constituent elements. Therefore, when a multicomponent metal oxide film is formed by ALD, the thin film formation process become complicated, and a complex manufacturing apparatus is required.
In order to solve the problems arising from using conventional CVD and ALD methods, a new method is disclosed in U.S. Pat. No. 5,972,430 issued to Dimeo in 1999. According to this method, the precursors containing the respective constituent metal elements of a target layer are simultaneously supplied into a reactor to form a precursor layer, and the reactor is purged with an inert gas. Next, all the metal atoms of the precursor layer are oxidized by an oxidizing agent, thereby resulting in a multicomponent metal oxide layer. Then, the reactor is purged again with an inert gas. This process cycle is repeated in order to complete the film formation. However, simultaneous supply of the precursors containing the respective constituent metal atoms in this method causes reaction between the constituent metal atoms, thereby lowering the vapor pressure. Precursors having low vapor pressure, such as Ba(thd)2 or TaCl5, are easily changed from a gas phase to a solid phase as the temperature-drops. This phase transition occurs easily when vaporized precursors are in transit states of being supplied into an ALD reactor. In addition, during the phase transition from vapor to solid, particles are easily generated during each transition going from a vapor state to a solid state. Once particles are generated, removing as well as handling of such particles is very difficult. Ba(thd)2 and Sr(thd)2, which are Ba and Sr precursors, respectively, react with an alkyl group precursor to form a compound having a low vapor pressure, which is difficult to handle. Here, the abbreviation “thd” stands for tetramethylheptanedionate ((CH)3CC(O)CHC(O)C(CH3)3−).
ALD methods, in which source gases containing the respective atoms constituting a target layer are separately supplied, can prevent reaction between the constituent atoms. However, when an organometallic compound is used as a precursor, the temperature of a substrate should be kept lower than the temperature of the organometallic compound because an organometallic compound is easily decomposed by itself at a high temperature, and easily form a solid state. Furthermore, if the temperature of the substrate is too low, the target layer cannot be formed because a desired reaction does not occur between supplied source gases. Thus, there is a need to maintain the substrate at a temperature greater than a minimal temperature at which formation of the target layer is caused. The range of minimum and maximum reaction temperatures of a substrate for ALD methods is varied depending on source materials. To form a multi-atomic layer by ALD, source materials containing the respective constituent atoms of the multi-atomic layer should have an overlapping range of minimum and maximum reaction temperatures. If a target layer is composed of many kinds of atoms so that there is no overlapping range of reaction temperatures between the constituent atoms, ALD methods cannot be applied to form the target layer. In addition, it is impossible to deposit a pure metal layer such as a Ti, W, or Al layer at a temperature lower than the thermal decomposition temperature of the metal layer by conventional ALD methods.
To form a layer with source materials having low reactivity, U.S. Pat. No. 5,916,365 to Sherman et al. discloses a thin film formation method involving supplying a first reactant gas into a reactor, exhausting the remaining first reactant gas from the reactor with a vacuum pump, supplying a second reactant gas which is activated passing through a radical generator such as a radio frequency (RF) power supply unit, and exhausting the remaining second reactant gas from the reactor with the vacuum pump. However, the pumping rate of the vacuum pump decreases as the pressure in the reactor decreases. Thus, it takes a considerable amount of time to fully exhaust the remaining reactant gas from the reactor.
To solve such problem, Korean Patent Application No. 99-11877 discloses a method of forming a thin film by generating plasma in synchronization with cyclic operation processes, which is incorporated herein by reference. The cyclic operation processes involve supplying a reactant gas, exhausting the remaining reactant gas from the reactor with a purge gas, supplying another reactant gas, and purging the reactor with a purge gas. Exhausting the remaining reactant gas using a purge gas is faster than using a vacuum pump. Thus, the method of Korean Patent Application No. 99-11877 can shorten the gas supply time with an improved film growth rate, compared to the method disclosed in U.S. Pat. No. 5,916,365. When plasma is directly generated in a reactor for a faster reaction rate, the method by Sherman et al. causes a serious change in gas pressure of the reactor, thereby destabilizing plasma. However, use of a purge gas can maintain the pressure of the reactor constant, thereby enabling stable plasma generation. In addition, to supply a solid material such as a barium (Ba) source used in the formation of a barium titanate layer at a constant rate, a liquid source supply unit for supplying a solution in which a solid material is dissolved in a solvent, and a vaporizer for converting a liquid source material to a gaseous form, are used. In this case, the method by Sherman et al. leads to clogging of the liquid source supply unit. In particular, when the reaction chamber is evacuated with a vacuum pump to purge a gaseous material that remains after reaction induced by supplying a reaction source, a highly volatile solvent rapidly evaporates near the supply line of the liquid source connected to the vaporizer. As a result, a viscous solid material remains in the liquid source supply unit and clogs the supply line of the liquid source. In contrast, the method disclosed in Korean Patent Application No. 99-11877 does not cause such a problem because the pressure of the reactor and the vaporizer is maintained at a constant level.